Acidic gases permeated carboxyalkyl starch and alkaline starch extrudates and process for making the same

ABSTRACT

The present disclosure relates to particles comprising carboxyalkyl starch that are permeated with an acidic gas and their uses as absorbent materials. It was discovered that superabsorbent materials could be obtained from carboxyalkyl starch particles permeated with the acidic gas and heated to a temperature of at least 100° C. until they reach an AUL at 0.7 psi. of at least 14 g/g and a CRC of at least 18 g/g. Moreover, it was discovered that the pH of alkaline starch extrudates can be adjusted by permeating particles of the extrudate with the acidic gas even with treating the particles to temperatures less than 100° C. The carboxyalkyl starch particles obtained by the methods described herein are characterized as having intramolecular ester bonds, which are greater in number at the surface of the particle than in the core, and the particles have a greater concentration of cation of the acidic gas at the surface than a the core.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry of PCT/US2010/024872 filedFeb. 22, 2010, which claims priority to U.S. provisional application No.61/154,119 filed Feb. 20, 2009, each of which are incorporated herein byreference in their entirety.

TECHNICAL FIELD

The present disclosure relates to acidic gas permeated particlescomprising carboxyalkyl starch and subsequent treatments thereof formaking bio based superabsorbent polymers. In particular, there isdisclosed a process for gas permeation and surface treatment ofparticles comprising carboxyalkyl starches. In a more particular aspect,there is disclosed a process for the neutralization of alkaline starchextrudates by permeating with an acidic gas. Also disclosed arecompositions of carboxyalkyl starches and alkaline starch extrudatesmade thereby, along with their uses and formulations.

BACKGROUND

Water absorbent materials, such as superabsorbent polymers, can beemployed in various applications such as in disposable hygiene articles(i.e. diapers, incontinence articles, feminine hygiene products,airlaids and absorbent dressings), household articles, sealingmaterials, humectants in agricultural products for soil conditioning, inoil-drilling fluids (i.e. lost-circulation material, fracturing fluids),anti-condensation coatings, in agricultural, horticultural and forestryapplications for retaining water in the soil and for the release ofwater to the roots of plants and trees, in the textile industry, inprinting applications, in absorbent paper products, in bandages andsurgical pads (i.e. wound dressings), in ore treatments, in concreteproducts, in pet litter, in water treatment, in food pads (i.e.applications related to the transportation of fresh food and foodpackaging), in detergents, in fire-fighting gels, in sealing materials,in cloud control, as chemical absorbents for the cleanup of acidicand/or basic aqueous spills including water soluble chemical spills, aspolymeric gels for the slow and controlled release of cosmetics andpharmaceuticals (also known as drug delivery systems) and finally in themanufacture of artificial snow. However, the primary use ofsuperabsorbent polymers, also referred as “SAPs”, is in disposablepersonal hygiene articles. Such products include, in decreasing order ofvolume of superabsorbent materials used, diapers, training pants, adultincontinence products and feminine hygiene products.

With the development of ultra-thin products, superabsorbent requirementsincreased. Not only superabsorbents need to absorb large amounts ofliquids, but they need also to retain those liquids under stress, swellunder pressure and even have a specific gel particle behavior whenswollen, as to permit liquids to flow. Among superabsorbents,polyacrylates are widely used today. But current polyacrylates are notbiobased, leading to increased carbon footprint, depletion ofnon-renewable oil reserves and increased vulnerability to energy pricingfluctuations. As an alternative, carboxymethyl cellulose (CMC) is partlybiobased and has long been for use as a superabsorbent material.

The major problem of CMC, however, lies in its excessive solubility inwater, which causes poor performance properties when deployed as asuperabsorbent material. Moreover, the manufacture of CMC typicallyresults in material with an unnecessarily high amount of substitution(carboxylation) per residue, i.e., greater than 0.7 substitutions perresidue. Because carboxylation requires the use of petroleum basedorganic reactants, the excessive carboxylation means increased materialcost, a lower degree of renewable matter, and increased carbonfootprint. Chatterjee et al.; U.S. Pat. No. 3,731,686; Reid et al.; U.S.Pat. No. 3,379,721 and Ning et al; U.S. Pat. No. 5,247,072 eachdescribed means to insolubilize CMC by heat treatment. Acidification hasalso been described as a means for insolubilization of by CMC Reid etal.; U.S. Pat. No. 3,379,720; Thornton et al.; U.S. Pat. No. 6,765,042and Kaczmarzyk et al.; U.S. Pat. No. 4,044,766. The major problem withthese types of acidification is the use of a liquid solvent as the acidcarrier requiring costly liquid handling step, additional energy to drythe solvent. In order to solve those problems, acidic gases had beenused to treat CMCs particles, such as described in Ouno et al. U.S. Pat.No. 3,391,135 and Marder et al. U.S. Pat. No. 4,200,737. However, CMCsstill have several drawbacks which made them unsuitable as absorbents.One major drawback is that highly absorbent CMCs are very specific tocertain types of cellulose fibers meaning that the manufacture of aconsistent product requires specific sources of cellulose fibre.Moreover, cellulose fibers from almost all natural sources are occur ina crystalline pattern that must be broken by the carboxymethylationreaction itself, resulting in differential and unpredictablesubstitution patterns through the cellulose polymer.

Carboxymethyl Starch (CMS) absorbents were far less investigated thanCMC. Gross U.S. Pat. No. 5,079,354 and Qin et al. U.S. Pat. No.5,550,189 described CMS absorbents. Due to water-based reactioninefficiencies or, alternatively, poor performances of dry or solventsynthesis, CMS was only reluctantly explored as a bio based absorbentmaterial. Theodorus et al. NL P 9100249A described CMS extrudates as apossible material for use absorbents. However, the process formanufacture described by Theodorus et al. used excesses ofmonochloroacetates to generate hydrogen chloride in-situ and resulted inmaterial with significant amounts of residual salts inside the CMSparticle, and the particles were uniformly acidified throughout ratherthan being surface treated as described in more detail herein after.Perhaps due to both the lack of surface treatment and the presence ofhigh amounts of salts, the CMS materials described by Theodorus et al.cannot reach acceptable industry specifications for use assuperabsorbent materials for diaper applications, such as having anAbsorbency Under Load (AUL) at 0.7 psi of at least 14 g/g and acentrifuge retention capacity (CRC) of at least 18 g/g. More recently,Koutlakis al. US App. 2008/177057A1 described a solvent based treatmentof CMS extrudates that resulted in CMS particles with an AULs of atleast 14 g/g. However, because those surface treatments were performedin solvent based systems, those processes have similar problems to thosedescribed by; Thornton et al.; such as additional liquid handling steps,additional energy costs and particle attrition, which was referred to inthat application as “static environment”.

The present disclosure addresses these problems and others, and providesfurther advantages that one of ordinary skill in the art will readilydiscern upon understanding the disclosure that follows.

SUMMARY OF THE INVENTION

The present disclosure refers to a number of documents, the content ofwhich are herein incorporated by reference only to the extent needed toprovide information and/or a source for materials and methods of makingto enable the production of CMS particles by the processes describedherein, or to understand terms of art used in the present disclosure,unless the incorporated references includes information that conflictswith the present disclosure, in which case the present disclosurecontrols and the information incorporated by reference shall be deemedvoid of the conflicting content.

It was unexpectedly discovered that superabsorbent polymers could beobtained from particles comprising carboxyalkyl starches that have beenpermeated with an acidic gas followed by a heat treatment. Thoseabsorbent materials could be done by a process comprising the steps of:permeating a particle comprising carboxyalkyl starch with an acidic gas;and. treating the particle to a temperature of at least 100° C.Optionally, the heating step is performed until the carboxyalkyl starchdevelops an AUL at 0.7 Psi of at least 14 g/g and a CRC of at least 18g/g.

Moreover, it was discovered that it was possible to adjust pH, evenneutralize alkaline starch extrudates particles with an acidic gas. Thisis also accomplished by permeating an alkaline starch extrudate particlewith an acidic gas. Typically, the alkaline starch extrudates are in theform of particles having a size ranging from 150 μm to 850 μm that mosttypically comprise carboxyalkyl starch, and in more typical embodimentscomprises carboxy methyl starch.

In a further embodiment, the present disclosure relates to the use ofsuperabsorbents made from acidic gas permeated particles comprisingcarboxyalkyl starch. Those particles may be used as absorbents indisposable sanitary products (i.e. diapers, incontinence articles,feminine hygiene products, airlaids and absorbent dressings), householdarticles, sealing materials, humectants in agricultural products forsoil conditioning, in oil-drilling fluids (i.e. lost-circulationmaterial, fracturing fluids), anti-condensation coatings, inagricultural, horticultural and forestry applications for retainingwater in the soil and for the release of water to the roots of plantsand trees, in the textile industry, in printing applications, inabsorbent paper products, in bandages and surgical pads (i.e. wounddressings), in ore treatments, in concrete products, in pet litter, inwater treatment, in cloud control, in food pads (i.e. applicationsrelated to the transportation of fresh food and food packaging), indetergents, in fire-fighting gels, in sealing materials, as chemicalabsorbents for the cleanup of acidic and/or basic aqueous spillsincluding water soluble chemical spills, as polymeric gels for the slowand controlled release of cosmetics and pharmaceuticals (also known asdrug delivery systems), as airlaids, and finally in the manufacture ofartificial snow. Those carboxyalkyl starches could also be used asabsorbents for liquids, non-limiting examples of which include water,aqueous solutions, physiological fluids and saline solutions.

In yet a further embodiment, the present disclosure relates tocompositions including acidic gas permeated particles comprisingcarboxyalkyl starch combined with another material. Those compositionstypically comprise the carboxyalkyl starch particles and a co-absorbentmaterial. Again, the most typical embodiments comprise carboxy methylstarch particles that have been treated with the acidic gas.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus generally described the invention, reference will now bemade to the accompanying drawings, showing by way of illustration apreferred embodiment thereof, and wherein:

FIG. 1 is a pH particle versus particle size distribution graph ofacidic gas permeated carboxyalkyl starch particles according to anembodiment of the present invention.

FIG. 2 illustrates a side elevation view of an extruder screwconfiguration used to manufacture carboxymethyl starches that aretreated according to an embodiment of the present invention.

FIG. 3 depicts a pH and HCl addition kinetics graph duringcarboxylmethyl starch cleaning, according to an embodiment of thepresent invention.

FIG. 4 is graph illustrating a pH adjustment of alkaline carboxymethylstarch extrudates permeated with various amounts of acid gas, accordingto an embodiment of the present invention.

FIG. 5 is a graph illustrating a relative concentration of chlorine bydepth of a particle comprising carboxymethyl starch treated with HCL gasaccording to an embodiment of the present invention. This was done by XRay Photoelectron Spectroscopy (XPS) with Argon etching.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS Definitions

In order to provide a clear and consistent understanding of the termsused in the present specification, a number of definitions are providedbelow. Moreover, unless defined otherwise, all technical and scientificterms as used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which the present disclosure pertains.

The use of the word “a” or “an” when used in conjunction with the term“comprising” in the claims and/or the specification may mean “one”, butit is also consistent with the meaning of “one or more”, “at least one”,and “one or more than one”. Similarly, the word “another” may mean atleast a second or more.

As used in this specification and claim(s), the words “comprising” (andany form of comprising, such as “comprise” and “comprises”), “having”(and any form of having, such as “have” and “has”), “including” (and anyform of including, such as “include” and “includes”) or “containing”(and any form of containing, such as “contain” and “contains”), areinclusive or open-ended and do not exclude additional, unrecitedelements or process steps.

As used in this specification and claim(s), the term “about” withrespect to a value means within the degree of error of an instrumentthat commonly would be used by one of ordinary skill in the art tomeasure the value in the context of this disclosure, and moreparticularly, within a range of the stated value where no discernablefunction or property would differ from the function or propertyexhibited precisely at the stated value. In non limiting embodiments forvarious parameters, the term may be within 10%, within 5%, within 1%,and in some cases within 0.5% of the stated value . . . .

As used in this specification, the term “percent” or “%” with respect toa material refers to a percentage by weight (i.e. % (w/w)), unlessotherwise specified.

As used in this specification, the term “saline solution” refers to a0.9% (w/w) sodium chloride solution in deionized water.

As used in this specification, the term “discrete particle” refers toindividual particles.

As used in this specification, the term “starch” refers to starchpolymers, its components and its derivatives, such as starches, modifiedstarches, amylopectin, modified amylopectin, amylose and modifiedamylose.

As used in this specification, the term “Free Swell Capacity” (FSC),also called “Total Absorption”, refers to the amount (g) of fluidabsorbed per gram of the composition. Typical fluids are salinesolutions (0.9% Weight/Weight NaCl solution, hereinafter called 0.9%NaCl solution or saline).

As used in this specification, the term “Centrifuge Retention Capacity”(CRC) also called “Retention” refers to the amount (g) of fluid retainedper gram of the composition, following exposure of the composition to acentrifugation force of 250G. Typical fluids are saline solutions.

As used in this specification, the term “Absorption Under Load” (AUL),at 0.7 PSI (5 kPa), also called “Absorption Against Pressure” (AAP) or“Absorption Under Pressure” (AUP) refers to the amount (g) of fluidabsorbed per gram of the composition under a given applied pressure.Typical fluids are saline solutions (0.9% Weight/Weight NaCl solution,hereinafter called 0.9% NaCl solution or saline).

As used in this specification, the term “absorbent material” or“absorbent polymer” refers to materials in a dry, solid state, havinggood fluid-swelling properties and capable of gel forming uponcontacting with a fluid. Non limiting examples of such fluids are water,aqueous solutions, saline, or physiological fluids.

As used in this specification, the term “superabsorbent”,“superabsorbent polymer” or “SAP” refers to absorbent materials capableof gel forming upon contacting with a liquid such as water, aqueoussolutions, saline, or physiological fluids. Such materials arecharacterized by a Centrifuge Retention Capacity (CRC) of at least 15g/g.

As used in this specification, the term “moisture content” refers to theamount of water (% w/w) contained in a solid.

As used in this specification, the term “granular material”, “granules”,“particles”, “powders”, “grains” or “dusts” refers to particulate matterin a finely divided state.

As used in this specification, the term “particle size” refers to thelargest dimension of a particle. The particle size can be directlydetermined using sieving methods, optical or scanning electronmicroscopes as well as by other well-known methods. Particle size isequivalent in meaning to the diameter of the particle if the particlewere perfectly spherical or the length of the particle if oblong.

As used in this specification, the term “discrete gel particles” refersto superabsorbent particles which, once swollen to their maximum extentin saline solution, have an appearance of discrete hydrogel particles.

As used in this specification, the term “particle surface” or “surfacezone” refers to the solid outermost layer of a particle. Thiscorresponds to a layer extending from particle's surface to a depth ofabout one third the particle size.

As used in this specification, the term “particle core” refers to thesolid innermost core of a particle. This core is located around theremotest point from the particle surface and extends to the inner mostboundary of the particle surface as defined above.

As used in this specification, the term “acidic gas” refers to amaterial in a gaseous phase that acts as an acid when in contact withhumidity or moisture. The acid definition means a Brønsted-Lowry acid,which is a compound able to donate a H⁺ under the conditions where theacid function is stated.

As used in this specification, “particle pH” or “carboxyalkyl starch pH”or “CMS pH” refers to the pH determined in an equilibrated 10%suspension of the particle in deionized water.

As used in this specification, “particle conductivity” or carboxyalkylstarch conductivity” or “CMS conductivity” refers to the conductivitydetermined in a 1% suspension of the particle in deionized water.

As used in this specification, “CMS” refers to carboxymethyl starch.

As used in this specification, an “extrudate” is a material formed by anextrusion process whereby an input stream of material in the form of asolid, a gel, an emulsion, suspension, or solution is submitted topressure and optionally to shear forces such as may be provided by animpeller or screw, so the material is pressed in a chamber against a dyehaving an orifice that permits the pressed material to emerge from thechamber in the form of a solid, a gel, an emulsion, or particle.

As used in this specification, “permeate” and grammatical variationsthereof, means to contact a material with a gas so that the gas spreadsover and through at least a portion of the material.

Carboxyalkyl Starch Particles

Among carboxyalkyl starches, carboxymethyl starch is usuallycontemplated. Carboxymethyl starch provides sufficient osmotic force,but also enough coulombic repulsion forces to achieve high absorbenciesunder load. In all cases described in this application, carboxymethylstarch should be considered as a typical carboxyalkyl starch.

Carboxyalkylation

Carboxyalkyl functionality may be easily grafted, via ether linkages, toalkalinized starches under a Williamson ether synthesis. This could beeasily done with reagents such as leaving groups bearing haloacids andsalts thereof. Non-limiting examples of such haloacids are C₂-C₅haloacids, such as monochloroacetic acid. Non-limiting examples of saltsthereof are alkali metals salts of haloacetic acids, such as sodiummonochloroacetate, potassium monochloroacetate, lithiummonochloroacetate and mixture thereof. A typical carboxyalkylationreaction could be summarized as follow.Starch-(OH)₃+X—(CH₂)_(y)—CO₂Z+WOH→Starch-(OH)_(3-m)—((CH₂)_(y)—CO₂Z)_(m)+mWX+H₂OWherein Y being the number of alkylene units. X being a nucleophilicleaving group, non-limiting examples of which are chlorine, bromine andiodine. W being an alkali metal. Z being an hydrogen, alkali metal orammonium group. m being the degree of substitution of the carboxyalkylstarch.

As contemplated by the present teachings, starch may be characterized asglucose polymer in alpha glycoside linkages with a molecular weight ofat least 500,000 g/mol. Starch could come from many sources.Non-limiting examples of starch sources are corn, wheat, potato, yam,cassava, rice, millet, sorghum, barley, oats, beans, favas, peas,lentils, buckwheat, bananas, arracacha, oca, sago, taro, sweet potatoes,waxy species thereof (such as waxy corn) and mixture thereof. Amongstarch sources, waxy corn, potato, corn and wheat are especiallycontemplated.

Among the methods for making carboxyalkylated starches, starchescarboxyalkylated after having been dispersed in an alkaline aqueousmedium are believed to be the most suitable choice. Without being boundto any theory, it is believed that carboxyalkylating agents, catalystsand starch chains are more labile in aqueous environment. Starchstructure is more easily penetrated by hydroxides and carboxyalkylatingagents, as starch is gelatinized and its semi-crystalline pattern isloosened. This gives the resulting effect that carboxyalkyl groups aremore evenly substituted and gives increased absorbent characteristics. Anon-limiting example of aqueous alkaline medium is an aqueousenvironment characterized by a pH of at least 11.0. Such pH could beachieved by dispersing an alkali hydroxide in water. Non-limitingexamples of such hydroxides are sodium hydroxide, lithium hydroxide andpotassium hydroxide. Typical moisture content in such aqueous alkalinemedium may range from 15% to 99%.

In a contemplated form of the present invention, the starch iscarboxyalkylated by a reactive extrusion process. This allowssubstantial increases in reaction efficiency and decreases reactiontime. Starch is an ideal substance for extrusion. In order togelatinize, starch requires a sufficient amount of water plus alkaliand/or heat. In reactive extrusion, the starch, water and alkali areadded in controlled amounts and after mixing, heat is applied to thereactive chamber of the extruder, allowing the starch to be carboxylatedand to gel only when needed, more specifically, near the extruder'skneading elements. This process not only limits unwanted side reactions,but also limits molecular weight degradation and reduces energyrequirements. The total water content in the carboxyalkylation-extrusionreactions typically ranges from 15% to 30%. This decreased amount ofwater, compared to solution based carboxyalkylation, provides higherreaction efficiency.

Typically, carboxyalkylation by reactive extrusion is performed using atwin screw extruder. Twin screw extruders provide the flexibility andthe shear necessary to perform carboxyalkylation with high efficiency atrelatively low moisture content. First, dry ingredients, such as starchand the carboxyalkylating agent are fed into the extruder. The dryingredients are conveyed to an alkali (hydroxide) injection point, whichis located nearer the kneading elements. The alkali is typicallyinjected as a hydroxide solution. Water may be optionally injected.Moisture typically reaches a content ranging from 15% to 30% of thereaction components in the extruder. In order to limit reagentdegradation in the extruder, temperature is conveniently kept at nogreater than 140° C. Under r these conditions, an alkaline aqueous doughcomprising carboxylated and gelatinized starch is produced. The dough isoptionally pumped into a die to obtain extrudate strands or pellets.Those extrudates are usually dried by convection means in a fluid beddrier to a moisture content ranging between 5% and 15%, which is neededto subsequently grind the extruded material into particles. Particlesizes ranging from 150 μm to 850 μm (20-100 Mesh) are desired fortypical superabsorbent applications.

pH Adjustment

Typically, after grinding into particles, the alkaline carboxylatedstarch extrudate is then permeated with an acidic gas. Particles may beplaced in a closed vessel where a partial vacuum could be produced todegas the particles prior to acidic gas permeation. Typically a vacuumof 30 kPa or lower is sufficient. After degassing, the acidic gas isadded to permeate the particles. Among acidic gases, gases of mineralacids are preferred. Halogen halides, and more specifically, hydrogenchloride are most typically used. Hydrogen chloride is a strong acid andhas a relatively low molecular weight (36.5 g/mol), allowing betterpermeation through the starch extrudate. Gas permeation may be done inone step or in multiple steps to ensure more thorough permeation of thematerial. In a multiple step permeation process, between each exposureto the acidic gas for a time sufficient to permeate the particles, theparticles are degassed in a partial vacuum to remove bubbles an preventformation of vapor blocking barriers that would interfere with thoroughpermeation. A moderate agitation may be done during the permeationprocess and/or the degassing steps. The temperature is usually keptunder 100° C., typically at room temperature (10° C. to 40° C.).

It was found that the pH of alkaline starch extrudates is advantageouslyadjusted by exposure to the acidic gas during permeation. This pHadjustment step has many advantages. The first advantage is to avoid, orreduce the need to adjust the pH during subsequent processing steps,such as solvent cleaning steps. Another advantage is the increasedpenetration of acid inside the alkaline starch extrudate particles, asthe relatively “dry chemistry” of the gaseous state Moreover salts aremore easily extracted from starch extrudate particles having a pHranging from 5.0 to 8.0. As mentioned herein before, typical starchextrudates are carboxyalkyl starch extrudates, especially carboxymethylstarch extrudates, which tend to form stronger salt bonds between thecarboxylate moieties and the cation of the salt at higher pHs.

It was found that grinding the alkaline starch extrudates into particleswill help both the neutralization and acidic gas permeation into thealkaline starch extrudates. In order to be ground, the starch extrudatesneed to have a moisture content of at most 12%. Moisture can be adjustedby drying. Once ground into particles, starch extrudates have a sizeranging from 150 μm to 850 μm. Particle size adjustment can be done bygrinding mills and sizes selected by sieving. Many types o grindingequipment can be used. Examples of suitable grinding mills are hammermills or roller mills.

Purification

Carboxyalkyl starch purity is an important consideration. Anysignificant amounts of residual impurities may lead to “salt poisoning”,which will cause performances reduction. To remove those salts, it istherefore typical to perform a purification step. The carboxyalkylstarch can be at least partially purified of salts by washing with waterand a water soluble organic solvent. Non-limiting examples of watersoluble organic solvents include C₁-C₄ alcohols and C₁-C₄ alcohol/watermixtures. Among C₁-C₄ alcohols, methanol, and more specifically,methanol/water mixtures are contemplated. Is it useful to keep the watercontent of such mixtures under the agglomeration threshold.Agglomeration threshold will cause carboxyalkyl starch particles toagglomerate and form masses during the cleaning step. Keeping underagglomeration threshold can be done by carefully selecting the waterconcentration in the solvent and controlling the temperature of thewashing step. Non-limiting examples are a 85/15 (v/v) methanol/watermixture at 60° C. or, a 75/25 (v/v) mixture at 22° C. Once the solventhas been used to clean the washed material, the carboxyalkyl starchparticles s are filtered and dried. The use of a “dryer” solvent, at theend of the solvent washing may ease drying, as it will remove water andprevent lump formation during subsequent steps. The dryer solvent mayalso be a water miscible organic solvent with less water content thanthe washing solvent, for example at least 90% methanol or ethanol.Carboxyalkyl starches may be considered purified when they comprise lessthan 1% sodium chloride or characterized as sufficiently cleaned of saltwhen a 1% suspension of the particles in deionized water has aconductivity of at most 1,500 μS/cm. The washing solvent and the dryersolvent may be recycled and reused by purification over an ion exchangeresin to remove the extracted salts.

After the washing step, is it usual to adjust the carboxyalkyl starchparticles moisture content and bulk density. Carboxyalkyl starchparticles are typically filtered from washing and drying solvents of thecleaning step. Upon drying, organic miscible solvents will usuallyevaporate before water. This will cause a relative moisture increase inthe carboxyalkyl starch particles, which will change the density of thecarboxyalkyl starch particle to a range from 0.5 to 0.7 g/cm³. Themoisture is also decreased to a content advantageously not greater than12%. It is possible that agglomerates could form during the drying stepdepending on the drying technique. The larger agglomerate s can berecovered after sieving and reground by grinding means, such as hammeror roller mills. The desired particle size, after the entire process istypically still from 150 μm to 850 μm (100 Mesh to 20 Mesh).

Permeation/Heating

Applicants surprisingly discovered that permeation of carboxyalkylstarch particles with gaseous acid, followed by a heat treatment at atemperature of at least 100° C. insolubilizes the otherwise solublecarboxyalkyl starch fraction of the particles. Even more interesting,applicants discovered that by a combination of proper heating time andtemperature, it was possible to cure the acidic gas treated particles inso that they reached an AUL at 0.7 Psi of at least 14 g/g, withoutdecreasing CRC below 18 g/g or FSC below 28 g/g.

Particles comprising carboxyalkyl starch to be permeated usually havemany characteristics, inferred from steps such as those describedpreviously. Particles comprising carboxyalkyl starch typically have abulk density usually ranging from 0.5 g/cm³ to 0.7 g/cm³. Typically,carboxyalkyl starches have a degree of carboxyalkyl substitution (asdetermined by ASTM D1439-83a method) ranging from 0.3 to 1.0 perresidue. Those having a degree of substitution ranging from 0.4 to 0.7are even more usual. Particles, when permeated, washed and driedaccording to the present teaching should have a moisture content of notgreater than 12%. A particle moisture content ranging between 0% and 8%is most suitable. Moisture content selection will adversely impactatmospheric pressure during the heat treatment step as will be describedherein later. Suitable carboxyalkyl starch particles made according tothe present teaching are characterized also by having pH ranging from5.5 to 7.5 when measured in a 10% (w/w) suspension with deionized watermore usually having a pH ranging from 6.2 to 6.8. They are relativelypure, typically containing less than 1% sodium chloride and arecharacterized by having conductivity of less than 1500 μS/cm as measuredin a 1% suspension with deionized water.

Carboxyalkyl starch particles comprising an even distribution ofcarboxyalkyl groups have promising structural characteristics. Indeed,because alkaline dispersion during synthesis provides for thecarboxyalkyl groups to be evenly substituted throughout the starch,carboxyalkyl groups are also found evenly distributed throughout thestarch particle core and particle surface. As acidic gas permeates theparticle catalyzing formation of esters with the carboxyl groups, thereis obtained an even distribution of esters in a gradient that has feweresters in the particle core than on the surface, making the surface morerigid and the core more porous thereby producing particles with higherCRCs.

It was surprising and advantageously discovered that permeation ofcarboxyalkyl starch particles with acidic gas in is not even throughoutthe particle. As mentioned herein above, the particles are typicallyselected to have a size from 150 μm to 850 μm. It was discovered thatafter the gaseous acid treatment, particles having a size ranging from150 μm to 250 μm have a lower pH (in a 10% w/w suspension in water) thanparticles having a size ranging from 600 μm to 850 μm. Particles havinga size ranging from 150-250 μm typically have a pH ranging from 4.80 to5.00 in that suspension while particles having a size ranging from 600μm to 850 μm typically have a pH ranging from 5.35 to 5.50. It isbelieved that a particle's core can be more easily penetrated by theacid permeation process when the particles are smaller (e.g. thicknesseffect), and therefore the measured pH in the suspension of smallerparticles is significantly lower than that for the larger particles.This phenomenon is best depicted in FIG. 1, which illustrates acorrelative relationship between particle size and pH, with a sharplylower pH being associated with smaller particles. Additionally, thisphenomenon was further characterized by X-Ray photoelectronspectroscopy, as depicted in Figure, which shows that the relativeconcentration of acidic gas anions (in this case chlorine) at thesurface of the particles is higher than the concentration in theinterior of the particle (the core) at a depth exposed after 5400seconds of Argon abrading time. In this case, the relative concentrationof chlorine at 5400 seconds of Argon abrading time is at least 10% lowerthan the relative concentration at zero seconds of abrading time.In-fact, FIG. 5 shows that the relative chlorine relative concentrationis relatively constant and 30% higher in the material exposed within thefirst 1500 seconds of Argon abrading time in comparison to the materialexposed at 5400 seconds and there is a regular correlation betweenparticle depth and chlorine concentration between these times.

As mentioned herein above, permeation may be conducted in multiple stepswith degassing in between. The information presented herein regarding pHand chlorine concentration provides a method for measuring effectivepermeation. Between each permeation step the pH may be assessed in a 10%(w/w) water suspension. Determination of pH values ranges from 4.5 to5.5, and more desirably from 5.3 to 5.5 indicates the permeation hasbeen successful and can be stopped when the pH reach those values.

Carboxyalkyl starch particles are allowed to react with the acidic gasand then degassed in a partial vacuum of 30 kPa or less, or moretypically 20 kPa or less for the heat treatment step. A partial vacuumof 3 kPa is typical, as it allows a wider flexibility in bothtemperature rise rate and initial moisture content. Pressure of 20 kPawill require more minute adjustments of temperature rise rate as well ashigher initial moisture content. A moderate agitation can be done duringthe permeation time, the reacting time, degassing time or the heattreatment time. Once degassed after permeation, and once thecarboxyalkyl starch has reached the target pH, the temperature is raisedto above 100° C. The hotter the carboxyalkyl starch particles are thelesser the time they need to be heat exposed to obtain suitableperformance properties. Heating is typically performed between 115° C.and 140° C. Heating time and temperature are sufficient when thecarboxyalkyl starch particles reach an AUL at 0.7 psi of at least 14 g/gand before their CRC decreases to under 18 g/g which will occur withprolonged heating or heating at too great of a temperature. Theparticles should also be optionally characterized by having a FSC of atleast 28 g/g.

Heating can be performed b direct conductive contact with the particleswith a heated gas by convection or by radiant contact. Typically,heating is performed in the same closed vessel used for permeation. Theheating source may be for example, an electromagnetic radiation source,a hot gas, a radiant heat element or a heated surface. Typically,infrared radiation sources identified as medium infra-red or carboninfra-red are also well suited.

In addition of absorbent performance characteristics, the carboxyalkylstarch particles will form an insoluble gel when swollen. This insolublegel will be made from a compendium of discrete gel particles. Becausethe gel particles are discreet and non agglomerated, aqueous solutionsare able to flow between and through the discreet gel particles allowingfor even distribution of the fluid throughout the compendium ofparticles. This is an especially desired feature of diapers, as liquidpenetration occurs through all axes of an absorbent core.

Without being bound to any theory, it is believed that the acidic gasacts as catalyst that accelerates a Fischer-esterification betweenstarch hydroxyl groups and the carboxylate groups of the carboxyalkylmoieties leading to intramolecular esterification between thesefunctional groups, particularly at the particle's surface. While theintramolecular esterification must cross link different portions of thestarch polymer, this is not the same surface cross linking as using across linking agent, because in the present teaching, no cross linkingagent is used, rather the HCL acid is merely acting as a catalyst tocause intramolecular esterification to occur. Surprisingly, theseintramolecular esters do not decrease centrifuge retention capacity tounder 18 g/g. The degree of intramolecular-esterification may becontrolled by various means, for example, by controlling the amount ofacidic gas used during permeation, controlling the pressure of acidicgas permeation process, varying the moisture content of the particle oracidic gas, varying the pressure during the heating treatment process,as well as controlling the temperature rise during the heat treatment.Because intramolecular esterification is catalyzed by the acid, and ithas been demonstrated that the acid does not uniformly penetrate theparticle, but rather penetrates the particle more at the surface thanthe core, it follows that the particles produced by acidic gaspermeation will also have fewer intramolecular esters formed in theinterior core than at the surface. Dissection of a sample of particlesand measurement of the esters formed in various sections woulddemonstrate a similar gradient of ester formation as the gradient ofchlorine distribution found and depicted in FIG. 5.

Ester formation can be determined by a variety of techniques. Onetechnique is to chemically measure ester bonds by dissolving 0.05 of theparticles in 1 ml of a solution of hydroxylammonium chloride. Then 4drops of 20% (w/w) NaOH solution are added and the mixture is brought to72° C. for 2 minutes and allowed to cool to room temperature (22° C.).Then, 2 ml of 1N solution of HCl is added. If the solution becomesmilky, 2 ml of ethanol (95% w/w) is added. Then dropwise, a solution offerrous chloride (5 g of FeCl₃ in 100 ml of deionized water) is added.Ester linkages are detected when the solution becomes purple and degreeof ester formation can be determined by spectrophotometric measurementof the evolved color.

The particles of the present disclosure may be mixed with otherco-absorbent materials to provide absorbent compositions. In anexemplary embodiment, the absorbent compositions may comprise from about1 to about 99% (w/w) of carboxyalkyl starch, and from about 99 to about1% (w/w) of co-absorbent material. Non-limiting examples of co-absorbentmaterials include synthetic absorbent polymers, starch-based absorbents,mannose containing polysaccharides, fibers and mixtures thereof.

Non-limiting examples of starch-based absorbents include glass-likestarches such as disclosed by Huppé et al. (CA 2,308,537); amylopectinnetworks such as disclosed by Thibodeau et al. (CA 2,462,053);polysaccharide agglomerates such as disclosed by Chevigny et al. (CA2,534,026); hydroxyethyl starch; hydroxypropyl starch; starchnanocomposites such as disclosed by Berrada et al. (CA 2,483,049); andmixtures thereof.

Non-limiting examples of mannose containing polysaccharides include guargum, tara gum, locust bean gum, konjac, mesquite gum, psyllium extracts,fenugreek extracts and mixture thereof. The mannose containingpolysaccharides may be chemically or enzymatically modified (i.e.mannose derivatives), cross-linked or in the form of nanocompositematerials.

Non-limiting examples of fibers include cellulose, viscose, rayon,cellulose acetate, polyamides (i.e. Nylon™), polyalkylenes,polyethylene, polypropylene, bi-component fibers, polyesters,polylactides, polypropanediols, polyhydroxyalkanoates, Lyocell™,sphagnum and mixtures thereof.

The synthetic absorbent polymers to be used as co-absorbent materials inthe absorbent compositions of the present disclosure, are generallyobtained from the polymerization, typically by radical or radical graftpolymerization, of monomers, non-limiting examples of which includeacrylic acid, acrylate salts, acrylic ester, acrylic anhydride,methacrylic acid, methacrylate salts, methacrylic esters, methacrylicanhydride, maleic anhydride, maleic salts, maleate esters, acrylamide,acrylonitrile, vinyl alcohol, vinyl pyrrolidone, vinyl acetate, vinylguanidine, aspartic acid, aspartic salts and mixtures thereof.

The particles of the present disclosure, or absorbent compositionscomprising such particles, are suitable for use in methods for absorbingliquids. In an embodiment of the present disclosure, one or more of thecarboxyalkyl starch particles of the present disclosure are contactedwith a liquid to be absorbed. Non-limiting examples of liquids includewater, aqueous solutions, physiological fluids and saline solutions. Theparticles of the present disclosure, upon contacting with the liquid(s)to be absorbed, will form a gel trapping the liquid(s) within.

The particles of the present disclosure could be used in hygienearticles, such as diapers, incontinence products, food pads and sanitarynapkins. The particles of the present disclosure may also be used inother applications such as in food pads, in agricultural, horticulturaland forestry applications for retaining water in the soil and for therelease of water to the roots of plants and trees; in the textileindustry, in printing applications, in absorbent paper products, in oretreatments, in concrete additives, in pet litter, in water treatment, incloud control, in drilling fluids (i.e. lost circulation materials,fracturing fluids); in food pads (i.e. applications related to thetransportation of fresh food and food packaging), in detergents,anti-condensation coatings, in fire-fighting gels; in sealing materials,in bandages and surgical pads (i.e. wound dressings); as chemicalabsorbents for the cleanup of acidic and/or basic aqueous spillsincluding water soluble chemical spills, as polymeric gels for the slowand controlled release of cosmetics and pharmaceuticals (also known asdrug delivery systems), and finally in the manufacture of artificialsnow. Particles could also be used as absorbents for liquids,non-limiting examples of which include water, aqueous solutions,physiological fluids and saline solutions.

Provided below are experimental protocols and examples to facilitate anunderstanding of the present disclosure and enable the production of theacidic gas permeated carboxyalkyl particles disclosed herein. Theseprotocols and examples are not by limitation, and are presented asexemplary only. Conditions and details may be modified, added to, ordeleted from these examples as selected by the skill of the one ofordinary skill in the art without departing from the essential teachingprovided herein.

Materials and Methods

Chemicals: Grade A wheat starch (Whetstar™ 4) was obtained from ArcherDaniels Midland (Decatur, Ill.). Sodium monochloroacetate granules wereobtained from Akzo-Nobel (Amersfoort, Netherlands). Sulfuric acid wasobtained from Fisher (Pittsburgh, Pa.). Sodium hydroxide, sodiumchloride, hydrochloric acid and methanol were obtained from Labmat(Quebec City, Canada). Hydrogen chloride was obtained from Air Liquide(Paris, France).

Equipment

Dextrinizer: A NOREDUX COOKER Type F/11 equipped with an internal heatedmixing shaft was used. Fluid bed dryer: A Carrier vibrating equipmentmodel QAD/C-1260 S was used to dry carboxymethyl starch extrudatespellets. Pelletizer: A Conair strand cutter was used to cut extrudatesstrands into pellets (about 1 mm thick, 3 mm diameter). Grinder: APallmann grinder type percussion PP8S was used. Convection oven: A Labtray drier TY 2, National Drying Machinery Company, (Philadelphia, USA)was used. Reactor: A 7000 liters double jacketed stainless steel batchreactor equipped with a shaft with 68 cm long propellers, spaced from 30cm from the reactor bottom, was used. Infra-red oven: A PanasonicNB-G100P infra-red oven was used. Lab grinder: A Braun™ model KSMgrinder was used to grind the samples when in small quantities.

Reactive Extruder

A Leistritz ZSE 40 HP (40 mm) twin screw extruder was used to forreactive carboxyalkylation. The extruder L/D ratio was of 40. Starch wasfed with an Acrison gravimetric agitated feeder (405-170-OE). Sodiummonochloroacetate was fed with an Acrison gravimetric feeder(405-1015-C). Starch and sodium monochloroacetate were fed between 30 mmand 180 mm. A sodium hydroxide injection nozzle was positioned at 560 mmfrom the beginning of the extruder, equipped with a Cole-Parmerperistaltic pump. A water injection nozzle was positioned at 720 mm fromthe beginning of the extruder, equipped also with a Cole-Parmerperistaltic pump. Closed side stuffer barrels were positioned between640 mm and 800 mm from the beginning. A vent was positioned between 1120mm and 1280 mm. The screw design is illustrated in FIG. 2 and detailedbelow.

Pitch length (mm) Element length (mm) Kneading block angle Extruder'sbeginning 20 mm 30 mm 60 mm 150 mm 30 mm 60 mm 45 mm 150 mm 45 mm 150 mm45 mm 50 mm 45 mm 50 mm 30 mm 60 mm Kneading block 60 mm 60° 6 elements(forward) Kneading block 60 mm 60° 6 elements (forward) 45 mm 30 mm 45mm 60 mm Kneading block 60 mm 90° 6 elements Kneading disc 10 mmKneading disc 10 mm 60 mm 150 mm 45 mm 150 mm 45 mm 60 mm 45 mm 60 mmExtruder's dischargeAll extruder's elements are double flighted. Kneading element thicknesswas 2 mm.Test Methods

As discussed in Modern Superabsorbent Polymer Technology (Buchholz, F.L. and Graham, A. T. Eds., Wiley-VCH, New York, 1998, section 4.6.1.Swelling Capacity: Theory and Practice, p. 147), several measurementmethods can be used to characterize the swelling capacity of a polymer.In the field of superabsorbents, the Gravimetric Swelling Capacity [alsocalled the Free Swell Capacity (FSC)] and the Centrifuge Capacity [alsocalled the Centrifuge Retention Capacity (CRC)] are recommended methods.The FSC and the CRC were used to characterize the swelling capacities ofthe obtained absorbent products.

Tea bags for FSC and CRC measurements: Tea bags (10×10 cm) were madefrom heat sealable Ahlstrom (Chirnside Duns, UK) filter paper (16.5±0.5)g/m² grade 07291. FSC measurements: The Free Swell Capacity (FSC) in a0.9% NaCl solution was determined according to the recommended testmethod WSP 240.2 (05) A from Worldwide Strategic Partners (EDANA-INDA).Tea-bag used was however slightly bigger, as described previously. CRCmeasurements: The Centrifuge Retention Capacity (CRC) in a 0.9% NaClsolution was determined according to the recommended test method WSP241.2 (05) A from Worldwide Strategic Partners (EDANA-INDA). The tea-bagused was however slightly bigger, as described previously. AULmeasurements: The Absorption Under Load (AUL) at 0.7 Psi, in a 0.9% NaClsolution was determined according to the recommended test method WSP242.2 (05) A from Worldwide Strategic Partners (EDANA-INDA). Petri dishtray had a bottom surface area of 177 cm², filter plate had a diameterof 90 mm and piston made from stainless steel. Those factors are notbelieved to have any significant influences on AUL measurements.

Carboxymethyl Starch Preparation

Wheat starch, having a moisture content of 10.0%, was fed into theextruder with an agitated gravimetric feeder in TSE (ZSE 40 mm), at athroughput of 13.1 kg/hr (28.8 lbs/hr). Sodium chloroacetate, was fedsimultaneously with a gravimetric feeder, at a throughput of 5.09 kg/hr(13 lbs/hr). A sodium hydroxide solution (50%) was injected, at athroughput of 4.26 kg/hr (9.4 lbs/hr). Barrels temperatures were B2=29°C., B3=29° C., B4=29° C., B5=43° C., B6=65° C., B7=82° C., B8=82° C.,B9=82° C., B10=82° C. Screw speed was set at 200 rpm and screw load at36-37%. TSE was equipped with a die comprising 10 holes of 3 mm ofdiameter. Die pressure discharge ranged from 620 kPa to 1.6 MPa (90-232Psig). An extrudate, having a temperature of 107° C. was conveyed to thepelletizer. The obtained pellets were then pneumatically conveyed to thefluid bed drier where they were dried at 80° C. for about 4 hours.Moisture content ranging from 8 to 11% was obtained. Samples were thenground in a hammer mill to 20-100 Mesh. An average DS of 0.55 wascharacterized according method ASTM D1439-83a.

Example 1 Surface Treatment of Carboxyalkyl Starches by Acidic GasPermeation

Ground and dirty CMS (320 Kg; 20-100 Mesh) was added into a 7000 literdouble jacketed stainless steel reactor. The reactor was then evacuatedto at 20 mm Hg. Nitrogen was injected to equilibrate with ambientpressure (760 mmHg). About 1725 liters of pre-heated solvent (MeOH/H₂O,at 82.5% MeOH), at 55-60° C. was injected in the reactor. The slurry wasagitated approximately at 100 rpm with a shaft equipped with 68 cm longpropellers spaced from 30 cm from the reactor bottom. The mixture pH wasadjusted by adding 2.1% (w/w HCl solution The HCl solution was typicallyprepared using 41.3 ml of HCl, 108.7 ml of tap water and 850 ml ofmethanol. Hydrochloric acid was added according to a kineticsillustrated in FIG. 3. As the data FIG. 3 was obtained with only 120 gof CMS (dry basis), 2373 times more HCl was used when the reaction wasscaled to a larger pilot scale reaction vessel with the same results.Cleaning was divided in 5 sub-washing steps lasting one hour each. Atthe end of each washing step, carboxymethyl starch particles wereallowed to settle and the supernatant liquid was discarded. A particlepH from 6.2 to 6.8 in a 10% suspension was measured at the end of thefifth wash. At the end of the fifth wash, the slurry was pumped with aWilden pump through 1-1½ pipes to a filter equipped centrifuge. Productwas centrifuged for 20 minutes at 400-600 rpm and the pelleted materialwas transferred into trays, producing a 3-4 cm thick layer of CMA anddried in a stainless steel oven at 60-70° C. for 3-5 days. Agglomerateswere obtained and broken in 3 cm pieces and ground to between 20-100mesh. From this product, 60 g was kept for Example 4.

This procedure was repeated twice. About 906 lbs of those two batchescombined was loaded in the dextrinizer. A 180 mm Hg vacuum was made for5 minutes under a mild mixing. About 1.8 kg (4 lbs) of hydrogen chloridewas then added and allowed to react about 30 minutes under a mildmixing. A particle pH ranging from 5.2 to 5.3 was then measured in a %in water suspension of the carboxymethyl starch sample. Then, thecarboxymethyl starch was re-vacuumed at 180 mm Hg. Still under mildmixing, the dextrinizer and carboxymethyl starch temperature was raisedto 121° C. in about 2 hours. Carboxymethyl starch was treated at 121° C.for 4.5 hours, still under mild agitation. Performances of this productwere summarized in Table 2:

TABLE 2 HCl permeated surface treated CMS FSC 31.4 g/g CRC 19.7 g/g AUL(0.7 psi) 14.6 g/gA sample similar to this one was further analyzed by X-Ray photoelectronspectroscopy. Samples were glued to a cooper disc, allowing them tocompensate due to their poor electric conductance. Being a poorconductor, a charge effect was observed at the particles' surface. Thiswas compensated by using a low energy source of electrons (10 eV). Evenafter this treatment, peaks stayed broad, but were notwithstandinganalyzed. Source was monochromatic, with an Al K alpha ray (1486.6 eV)produced by de 10 kV e−, P=150 W. The beams were focused on a spot(1000×750 μm) with LAXL lenses. Sample was ion etched with Ar, 3 kV,with a surface area etched of (1.5 mm×2.5 mm) and with an etching rateof about 25 nm per minute for carbon materials. The pressure was ofabout 1×10⁻⁹ mbar. The chlorine concentration measured under his Argonabrading technique is depicted in FIG. 5.

Example 2

pH Adjusted and Surface Treated Carboxymethyl Starches by Acidic GasPermeation

Carboxymethyl starch from the extruder (3980 kg) was loaded in thedextrinizer. The dextrinizer was kept under a 180 mm Hg vacuum with agentle rotation agitation for 5 minutes. 4.54 kg of gaseous hydrogenchloride was added inside the dextrinizer at a rate of 0.45 kg/min.After adding the gaseous HCl portion, the product was kept under gentleagitation for 5 minutes and a vacuum of 180 mmHg was made. A sample ofcarboxymethyl starch particles was taken from the dextrinizer todetermine its pH in 10% water suspension.

Another 4.54 kg of gaseous hydrogen chloride was added inside thedextrinizer at a rate of 0.45 kg/min. After adding the gaseous HClportion, the product was kept under gentle agitation for 5 minutes andthe 180 mmHg vacuum was made. A sample of carboxymethyl starch particleswas taken from the dextrinizer to determine its pH in 10% watersuspension. Another 4.54 kg of gaseous hydrogen chloride was addedinside the dextrinizer at a rate of 0.45 kg/min. After adding thegaseous HCl portion, the product was kept under gentle agitation for 5minutes and the 180 mm Hg vacuum was made. A sample of carboxymethylstarch particles was taken out from the dextrinizer to determine its pHin 10% water suspension. To this stage, a total 13.62 kg of hydrogenchloride was first added and a final 2.27 kg of hydrogen chloride wasfinally added. The product was subject to a vacuum at 180 mm Hg anddischarged from the dextrinizer. Its final pH was 6.27 as measured in a10% water suspension. All carboxymethyl starch particles samples weremeasured for their pH. Measurements are shown in FIG. 4.

From the neutralized, but dirty mass of CMS of Example 1, 1.2 kg of CMSwas placed in 6000 ml of 85% (v/v) methanol/water solution at 60° C. for60 minutes. Product was filtered and placed again, for a second time, in6000 ml of a 85:15 (v/v) methanol/water solution at 60° C. for 60minutes. Product was filtered and placed again, for a third time, in6000 ml of a 85:15 (v/v) methanol/water solution at 60° C. for 60minutes. Product was filtered and placed again, for a fourth time, in6000 ml of a 85:15 (v/v) methanol/water solution at 60° C. for 60minutes. Product was filtered and placed again, for a fifth time, in6000 ml of a 85:15 (v/v) methanol/water solution at 60° C. for 60minutes. A sample was taken for measurements. A 10% in water suspensionhaving pH of 7.37; a NaCl content of 0.11% and a conductivity of 835μS/cm was recorded. Resulting solids were then filtered and dried in aconvection oven at 65° C. The product formed cakes that were ground to20-100 mesh.

From the ground mass, 40 g was placed in a 500 ml polypropylene jarwhich could be closed with a septum equipped lid. A 150 ml polypropylenebeaker was placed over the samples and filled with 10 g of NaCl. Asyringe comprising 1.5 g of concentrated sulphuric acid was injectedover the NaCl beaker, generating hydrogen chloride. This mixture wasallowed to react 10 minutes. Thereafter, the pH of the CMS was measuredand sulphuric acid was added this way, until the CMS reached a pH of5.43 in a 10% water suspension. From the resulting CMS, 10 g was placedin a 9 cm crystallizing pan. The CMS was IR heated for 20 minutes at140° C. Performances of this product are summarized in Table 3:

TABLE 3 HCl permeated surface treated CMS FSC 31.5 g/g CRC 19.8 g/g AUL(0.7 psi) 14.8 g/g

Example 3

pH by Particle Size

From the cleaned and ground mass of Example 2, 60 g was placed in a 500ml polypropylene jar which could be closed with a septum equipped lid. A150 ml polypropylene beaker was placed over the samples and filled with10 g of NaCl. A syringe comprising 0.6 ml of concentrated sulphuric acidwas injected over the NaCl beaker, generating hydrogen chloride. CMSparticles were sieved on 1180 μm, 850 μm, 600 μm, 425 μm, 300 μm, 250 μmand 150 μm and the pH of these size fractionated particles is shown inFIG. 1.

What is claimed is:
 1. An absorbent particle comprising a carboxyalkylstarch that has been permeated with an acidic gas, wherein said particlecomprises an inner core region and an outer surface region surroundingthe core region, and has a relative amount of intramolecular ester bondsthat is evenly dispersed at the surface region and at a greaterconcentration at the surface region than the inner core region as aconsequence of acidic gas-permeation, wherein the intramolecular esterbonds do not comprise a cross linking agent.
 2. The absorbent particleof claim 1, wherein the carboxyalkyl starch is carboxymethyl starch. 3.The absorbent particle of claim 1, wherein the acidic gas is hydrogenchloride.
 4. The absorbent particle of claim 1, wherein a pH measured ina solution consisting of water and an amount of the surface of theparticle is lower than the pH measured in a solution consisting of waterand the same amount of the inner core of the particle in a 10% (w/w)suspension of the particles in deionized water.
 5. The absorbentparticle of claim 1, wherein said absorbent particle exhibits anabsorption under load (AUL) at 0.7 psi of at least 14 g/g and acentrifuge retention capacity (CRC) of at least 18 g/g with a solutionof 0.7 M NaCl.
 6. The absorbent particle of claim 1, wherein saidabsorbent particle forms discrete gel particles upon swelling withwater.
 7. The absorbent particle of claim 1, wherein the relative amountof intramolecular ester linkages is indicated with a ferrouschloride/hydroxylamonium test that indicates an amount of ester bondsoccurring in the same weight of material taken from the surface of theparticle and the inner core region of the particle.
 8. The particle ofclaim 1, wherein said particle comprises an anion obtained fromdissociation of the acidic gas.
 9. The particle of claim 8 wherein theanion is chlorine.
 10. The particle of 9, wherein the relative chlorineconcentration is higher at the surface of the particle than in the innercore of the particle when measured by zero second Argon abrading time incomparison to 5400 second Argon abrading time as determined by XPS withthe same amount of material taken from the surface and the inner coreregion of the particle.
 11. The particle of claim 10, wherein thechlorine relative concentration is higher than 30% within the first 1500seconds of Argon abrading time than at 5400 second Argon abrading timeas determined by XPS.
 12. The particle of claim 1, wherein said particlehas: a size ranging from 150 μm to 850 μm; a bulk density from 0.5 g/cm³to 0.7 g/cm³; and a moisture content no greater than 12%.
 13. Theparticle of claim 1, wherein the carboxyalkyl starch is furthercharacterized by at least one characteristic selected from the groupconsisting of: a degree of carboxyalkyl substitution ranging from 0.3 to1.0, an even distribution of carboxyalkyl groups, a pH ranging from 5.5to 8.0 when measured in a 10% (w/w) suspension of the particles indeionized water, a NaCl content of no greater than 1% (w/w), and aconductivity of no greater than 1500 μS/cm 1 when measured in a 1% (w/w)suspension in water.
 14. An absorbent product comprising; the absorbentparticle of claim 1 in combination with a co absorbent material.
 15. Theabsorbent product of claim 14 selected from the group consisting ofdiapers, incontinence articles, feminine hygiene products, printingproducts, textile products, absorbent paper products, airlaids,absorbent dressings, household articles, sealing materials, humectants,anti-condensation coatings, soil conditioning products, litter products,concrete products, oil-drilling fluids, mining fluids, chemicalabsorbents, controlled release polymeric gels, detergents, fire-fightinggels, artificial snow, and food pads.
 16. A method for absorbing aliquid comprising contacting the liquid with the absorbent particles ofclaim 1, wherein the liquid is selected from the group consisting ofwater, aqueous solutions, physiological fluids and saline solutions.